The dust particle removal means of conventional excimer laser apparatus are available in two types, one of which uses a filter (refer to Japanese Published Unexamined Patent Application (A) 4-137574; "Control Of Contaminants In XeCl Lasers", LASER FOCUS WORLD, p. 65-68, October 1981; and others), and the other of which uses a static dust particle remover (refer to Japanese Published Unexamined Patent Application (A) 58-186985; U.S. Pat. No. 5,027,366; and U.S. Pat. No. 5,029,177).
As regards the filters, stainless steel mesh filters which are anti-corrosive to fluoric gas have been used. However, if these mesh filters are used in an excimer laser apparatus which employs a fluorine based gas (mainly, XeF, KrF and ArF), chromium on the surfaces of the stainless steel fibers which form the mesh reacts with F.sub.2 contained in a laser medium gas to produce a chromium fluoride. A low value chromium fluoride, for example, CrF.sub.2, remains as is on the surfaces of the stainless steel filters, as its vapor pressure is high. However, a high value chromium fluoride, for example, CrF.sub.5, is mostly a gas at the temperatures of a normal laser medium gas, as its vapor pressure is high, and is therefore mixed with the laser medium gas. In laser oscillation to be carried out with a laser medium gas containing CrF.sub.5, any gaseous CrF.sub.5 near the windows which transmit the laser beam is decomposed by the ultraviolet radiation of the excimer laser, and Cr is caused to be deposited on the windows. Consequently, there will be a problem in that the Cr deposits will cloud the windows, lowering the transmission ratio of the laser beam through the windows and reducing the output of the laser beam.
On the other hand, in case of the filters made of an organic material, for example, polytetrafluoroethylene filters, there is a problem in that water and the fluorine in the polytetrafluoroethylene react to produce HF, and CF.sub.4 is produced by the laser beam, thereby reducing the laser output.
As shown in FIGS. 10 and 11, a conventional static dust particle remover comprises a dust particle removal case 21, a case cover 22, an inlet 23, an outlet 24, wire electrodes 25 to which a high voltage is applied, cylindrical electrodes 26 having a ground potential, wire electrode support plates 27, current introduction terminal blocks 28 and 29, a stabilizing resistor 31, and a high voltage power supply 32. A high voltage current from the high voltage power supply 32 is introduced into wire electrodes 25 through the stabilizing resistor 31 and the current introducing terminal 29. The four wire electrodes 25 are connected to the wire electrode support plate 27 at the front side and to the wire electrode support plate 27 at the rear side, so that they are in parallel with each other, with each wire electrode 25 having first and second portions connected in series, with the first portion being in a front static dust particle remover and the second portion being in a rear static dust particle remover, as shown in FIG. 10. The front cylindrical electrodes 26 are connected to the dust particle removal case 21 at the front side, and the rear cylindrical electrodes 26 are connected to the dust particle removal case 21 at the rear side. A laser medium gas containing dust particles enters into the dust removal case 21 through the inlet 23 provided at the central position of the dust removal case 21, and dust particles are collected by four cylindrical electrodes 26 and wire electrodes 25 which are arranged in parallel connection. Then the gas is discharged from the outlet 24.
When such cylindrical static dust particle removers are operated for a long period of time, wire electrodes 25 will become disconnected due to wear or unable to discharge due to an insulation layer formed by the deposition of dust particles, and therefore dust removal will not be carried out. If a wire electrode 25 is disconnected, two types of results are expected, depending on the pattern of connection after the point of disconnection. When a broken wire electrode 25 comes into contact with a cylindrical electrode 26, the protection circuit of the high voltage power supply 32 operates to prevent the high voltage from being applied to the other wire electrodes 25. When a broken wire electrode 25 does not come in contact with any other part, the associated cylindrical electrode 26 does not function as a static dust particle remover, and all dust particles which have passed through the associated cylindrical electrode 26 enter into the clean side. The detection of any abnormality in this case is difficult and, even though the abnormality of the static dust particle removers can be detected by providing an abnormality detector, the cross flow fan which circulates the gas does not stop immediately, and therefore the clean side of the static dust particle removers can become contaminated. When this phenomenon occurs, it is necessary to clean off all dust particles deposited at the clean side, and it takes a long period of time to recover the normal condition.
The operating conditions of the static dust particle removers vary with the degree of accumulation of dust particles in the static dust particle removers, along with the increase of operating time of the excimer laser apparatus. For this reason, the static dust particle removers are operated according to the operating condition as the maximum common measure obtained in view of a time dependent change of accumulated dust particles. However, this method makes it difficult to operate the static dust particle removers with satisfactory efficiency of dust collection throughout the overall operating time. Particularly, in the latter half of the service life of the static dust particle removers, when much dust has been deposited thereon, the quantity of dust particles which cannot be collected by the static dust particle removers and which accordingly passes to the downstream side will increase. The static dust particle removers should be large-sized to prevent such undesirable accumulation of dust particles.
In addition, the contamination of the optical system is one of the detrimental factors to the extension of the service life of the laser oscillation. When dust particle compounds or the like, produced by repeated laser oscillation, deposit on the window, the transmission ratio of the laser beam is reduced and the laser output deteriorates. The inventions disclosed in Japanese Published Examined Patent Application (B2) 60-26312 and U.S. Pat. No. 5,018,161 specify that a gas, purified through the filters, be forcibly blown against the window surfaces, and therefore require a great volume of purging gas to prevent involving the laser medium gas containing dust. Meantime, the invention disclosed in Japanese Published Unexamined Patent Application (A) 58-186985 specifies a system for cleaning the windows with a clean gas by providing a gas inlet for purified gas in the window areas and a gas circulating passage which reaches the gas inlet through the gas outlet of the discharge tube to the electric dust collector and the standstill zone. However, the gas circulating passage is installed outside the discharge tube and therefore there is a possibility of an increase in the cavity length and leakage from piping connections. Moreover, the above invention includes a problem in that the aperture mask of the conventional apparatus provided outside the chamber reacts with the ultraviolet radiation and the atmospheric gas to result in deterioration, or impurities are produced to contaminate ambient optical components.